1. Field of the Invention
This invention relates to the treatment of oil wells, gas wells, injection wells and similar boreholes. In one aspect it relates to a method of stimulating the productivity of hydrocarbon-bearing formations by hydraulic fracturing techniques. In a more specific aspect, it relates to a method of preventing the overdisplacement of propping agent particles into a subterranean formation during the hydraulic fracturing treatment.
2. Description of the Prior Art
A common technique for stimulating the productivity or injectivity of subterranean formations is a treatment known as hydraulic fracturing. In this treatment, a fluid is injected down the well and into the formation at a high pressure and rate to cause the formation to fail in tension, thereby creating a crack (fracture) in the formation. The earth stresses are normally such that the fracture is vertical, extending in opposite directions from the well. The fracture can be extended several hundred feet into the formation depending upon the volume and properties of treating fluid. The fracture is normally propped open by means of particles known as propping agents. The propping agent is carried down the well and into the formation as a suspension in the fracturing fluid. As the fracturing fluid bleeds off into the formation, the propping agent is deposited in the fracture. Upon the release of the fluid pressure, the fracture walls close upon the propping agent. The propping agent thus prevents the fracture from completely closing, thereby creating a highly conductive channel in the formation. If properly performed, the hydraulic fracturing treatment can increase productivity of a well several fold.
A problem associated with the placement of the propping agent in a fracture is that of overdisplacement. As pointed out in SPE Paper 3030 "Stresses and Displacements Around Hydraulic Fractured Wells" published by the Society of Petroleum Engineers of the AIME in 1970, the closure stress of a fracture at the mouth in the near wellbore region can affect productivity. If the fracture is not completely filled with propping agent in the near wellbore region, the productivity will be greatly reduced. Studies have shown that the stress level in this region causes the fracture to close upon incomplete fracture fill-up, thereby reducing the effectiveness of the treatment.
On the other hand, if too large a volume of propping agent is used, the process will settle in the wellbore and could cover the well perforations and reduce well productivity.
The normal technique for preventing overdisplacement of the slurry (propping agent particles suspended in the fracturing fluid) is to carefully monitor the volume of fluid pumped into the well so that upon injection of the proper volume of displacement fluid, the pumping operations are terminated. The proper displacement volume is based upon tubular volume calculations. However, the instruments, including flowmeters, tank strapping techniques, etc., used to measure the total volume of displacement fluid are not precise. Because of the inherent inaccuracies in these instruments, the monitoring technique frequently results in underdisplacement or overdisplacement of propping agent into the fractures.